The quality of ready mix concrete is very important and following are the important parameters to be considered in the manufacture of good quality ready mix concrete.

Choice of admixture:
  • Choose Admixture that gives initial workability as per the requirements and very good workability retention- The slump drop shall not be more than 30mm per hour. The admixture should be tested for compatibility with cement.
  • Note: Avoid Admixture with air entraining effect as this leads to drastic drop in the workability, arresting of bleeding and drying of concrete surface at faster rate.

Workability criteria:
  • Design the mix in such a way that it has a workability of not less than 120mm when Transit Mixer reaches the site. 
  • Note: The usual duration for the process - pumping (10 cum/hr), compaction, levelling and finishing - of the concreting takes about 1 hour, so by the time last one cum is cast and finished it shall have the workability that does not allow concrete surface to dry fast. This is essential for other valid reason also, which is, to give site-engineer sufficient time to look back at the portion of slab cast already and start the early curing during and after the process of concreting.
Water content:
  • Chose an optimal water content that is required to wet the surface of aggregates to give the sufficient workability. 
  • Unless High Grade Super plasticizers with good slump retention are used it is very difficult to achieve design workability of 120mm at site (2 hours after batching).
Control over aggregates
  • Coarse aggregate shall be controlled for the presence of dust content. Fine aggregate shall be controlled for the presence of clay content completely and to limit the silt presence by less than 6%, by volume as clay and silt cause loss of workability. 
  • When M.Sand / CRF is used it shall be controlled for excess presence of fines passing through 0.300mm sieve and limit the fines passing through 0.075mm to zero. 
  • Wetting the Aggregates: Aggregates in the stock pile shall be made wet by sprinkling of water. This will not only control the absorption rate but concrete temperature too.

Will be Continued in Issue 6.. 
Published by:
Task Force for Quality Assurance in Public Constructions

Important Note:
This reference manual is intended for the use of Government Engineers.
Every effort has been made to avoid errors or omissions in this publication. In spite of this, some errors might have crept in. Any mistake, error or discrepancy noted may be brought to our notice which shall be taken care of in the next edition. It is notified that neither the publisher nor the authors will be responsible for any damage or loss of action to any one, or any kind, in any manner, therefrom.
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